Nitride semiconductors have the characteristics of high saturated electron velocities, wide band gaps, etc. Therefore, various studies are conducted for applying nitride semiconductors to high-breakdown-voltage and high-output semiconductor devices by utilizing these characteristics. For example, GaN which is one of the nitride semiconductors has a band gap of 3.4 eV larger than the band gap (1.1 eV) of Si and the band gap (1.4 eV) of GaAs. Therefore, GaN has high breakdown electric-field strength and is very promising as a material for semiconductor devices for power supply which is capable of achieving high-voltage operation and high output.
With respect to semiconductor devices using nitride semiconductors, there have been many reports on field-effect transistors, particularly high electron mobility transistors (HEMT). For example, as GaN-based HEMT, AlGaN/GaN-HEMT using GaN for an electron transit layer and AlGaN for an electron-donating layer attracts attention. In the AlGaN/GaN-HEMT, distortion due to a difference in lattice constant between GaN and AlGaN occurs in AlGaN. Thus, a high concentration of two-dimensional electron gas (2DEG) may be produced by piezopolarization due to the distortion and spontaneous polarization of AlGaN. Therefore, the AlGaN/GaN-HEMT is expected as a high-efficiency switch element, a high-breakdown-voltage power device for electromobiles, and the like.
However, a GaN substrate having good crystallinity is very difficult to produce. Therefore, nitride semiconductor layers, for example, a GaN layer and an AlGaN layer, are usually formed by heteroepitaxial growth on a Si substrate, a sapphire substrate, or a SiC substrate. In particular, the Si substrate is easily available at low cost with a large diameter and high quality. Therefore, studies are being actively conducted on a structure in which a GaN layer and an AlGaN layer are grown above a Si substrate.
However, usual GaN-based HEMT using a Si substrate has difficulty in suppressing a leak current flowing between a source and a drain.
Japanese Laid-open Patent Publication No. 2008-235738 discusses a GaN-based semiconductor element, and Japanese Laid-open Patent Publication No. 2010-103236 discusses a nitride semiconductor device.